Method of and system for calibrating a repeater

ABSTRACT

A method of and system for calibrating a repeater in a wireless communications system are provided. The one or more calibration parameters for the repeater are derived from (a) time measurements derived from one or more signals relayed by the repeater and received at a plurality of different measurement positions, and (b) the positions of the measurement locations. In one application, the one or more parameters are used in determining the positions of subscriber stations in the wireless communications system.

RELATED APPLICATIONS

[0001] This application claims the benefit of provisional U.S.application Ser. No. 60/408,611, entitled “Method of and System forCalibrating a Repeater,” filed on Sep. 5, 2002, assigned to the assigneeof the present application, and incorporated herein by reference in itsentirety for all purposes.

FIELD OF THE INVENTION

[0002] The disclosed subject matter relates to the fields of positiondetermination and wireless communications, and more specifically, toposition determination in a wireless communications system that employsrepeaters.

RELATED ART

[0003] Repeaters are a flexible and cost effective way to extendcoverage or fill coverage gaps in wireless communications systems. Theymay also be used to render a particular pilot dominant within a coveragearea of a CDMA system that is subject to many pilots. Examples of areaswhere repeaters typically prove useful for adding or extending coverageare terrain variations such as valleys, tunnels and buildings. For asmall fraction of the cost of full base stations, repeaters areappropriate for use in new as well as well-established and maturenetworks.

[0004] There are several implementations of repeaters. In the mostcommon implementation, the repeater acts as a bi-directional amplifieras shown in FIG. 1. On the forward (down) link, the repeater 104receives a transmission from donor cell base station (BTS) 102,amplifies it and sends it to the subscriber station (SS) 106. The BTS102 may be an omni station or a sector in a multi-sector cell. On thereverse link, the SS 106 sends a signal to the repeater 104. Therepeater 104 amplifies the signal and sends it to the BTS 102. As can beseen, the repeater relays a reliable signal between the donor cell andthe subscriber station in an area that may not otherwise have reliablecoverage.

[0005] Pursuant to an FCC mandate, efforts are underway to equipsubscriber stations with the capability of determining their locationsfrom transmissions received from various reference sources, such as GPSsatellites, base stations, or combinations of GPS satellites and basestations. The subscriber station receives transmissions from four ormore references sources whose precise positions are known. Thesereference sources are synchronized to system time. The subscriberstation then derives a time measurement from each of the transmissions.The time measurement represents the amount of time required for thesignal to travel along a line-of-sight path between the reference sourceand the subscriber station. This time is commonly referred to as the“propagation time”. The subscriber station then provides these timemeasurements to a position determination entity (PDE). In response, thePDE estimates the location of the subscriber station from (a) these timemeasurements, (b) the known speed of light, and (c) the known locationsof the reference sources. Alternatively, the subscriber station usesthis information to determine its own position.

[0006] The presence of repeaters in a wireless communications system canrender the position determination process ambiguous. For example, due tothe presence of repeaters, there is a danger that a subscriber stationwill erroneously assume a transmission originating from a base stationbut relayed by a repeater is a line-of-sight transmission from the basestation. Since a time measurement derived from this transmission willoverstate the propagation time between the base station and thesubscriber station, an estimate of the location of the subscriberstation based on this time measurement will be erroneous.

[0007] This problem may be further explained with reference to FIG. 2.As shown, subscriber station 212 receives transmissions from fourreference sources, comprising GPS satellite 202, BTS 204, GPS satellite206, and BTS 206. Each of the transmissions from sources 202, 204, and206 is a line-of-sight transmission. However, there are twotransmissions received from BTS 208. The first, identified with numeral214, is received directly from BTS 208. The second, identified withnumeral 216, is routed through repeater 210. Both transmissions from BTS208 are modulated with the same PN code uniquely identifying BTS 208.The transmission 216 routed through the repeater 210 is stronger thantransmission 214, and hence is chosen by the subscriber station 212 foruse in the position determination process in lieu of the transmission214.

[0008] The subscriber station 212, upon receiving the transmissions,erroneously identifies the transmission 216 relayed by repeater 210 as aline of sight transmission from BTS 208. Therefore, it also erroneouslyidentifies the time measurement derived from this transmission as beingrepresentative of the propagation time between the BTS 208 and thesubscriber station 212. However, this time measurement is notrepresentative of this propagation time, but in fact overstates it.Consequently, a position estimate based on this time measurement will beerroneous.

SUMMARY

[0009] A method of calibrating a repeater in a wireless communicationssystem is described. In one embodiment, the method begins by receiving asignal at each of several different measurement locations. Each signaloriginates from the same originating transmitter and is relayed by therepeater prior to being received at the measurement location. Timemeasurements are then derived from each of the signals. Each of the timemeasurements represents the time between transmission of the signal atthe originating transmitter, and arrival of the signal at themeasurement location. The positions of the measurement locations areeither known or obtained. Calibration parameters for the repeater arethen derived from (a) the time measurements, and (b) the positions ofthe measurement locations.

[0010] In one implementation, the calibration parameters for therepeater comprise (a) a time correction for the repeater, and (b) theposition of the repeater. Both parameters are derived throughapplication of an inverse triangulation procedure to time measurementsderived from signals received at four different measurement locations.The signals all originate from the same donor base station and are eachrelayed by the repeater before being received at the measurementlocations.

[0011] The time measurements derived at these four locations may bereferred to as m_(i), where 1≦i≦4. Each time measurement mi can beexpressed as:

m _(i)=Δ+τ₁+τ_(R)+τ_(2i)   (1)

[0012] where:

[0013] τ_(i)=the forward link delay between the originating transmitterand the repeater

[0014] τ_(R)=the repeater self delay

[0015] τ_(2i)=the forward link delay between the repeater and thereceiver at measurement location i

[0016] Δ=the offset between system time and time at the originatingtransmitter

[0017] Assuming that the position of the repeater has the unknowncoordinates (x_(R), y_(R), z_(R)), and the position of measurementlocation i has the known coordinates (x_(i), y_(i), z_(i)), therelationship between the time measurement m_(i), the coordinates of theposition of the repeater, and the coordinates of the position ofmeasurement location i may be expressed as follows: $\begin{matrix}{m_{i} = {\Delta \quad + \tau_{1} + \tau_{R} + {\frac{1}{c}\sqrt{\left( {x_{i} - x_{R}} \right)^{2} + \left( {y_{i} - y_{R}} \right)^{2} + \left( {z_{i} - z_{R}} \right)^{2}}}}} & (2)\end{matrix}$

[0018] where c is the speed of light.

[0019] Note that (2) represents four separate equations, one for each ofthe measurement locations. These four equations can be solved for fourunknowns. The first three unknowns are the coordinates of the positionof the repeater (x_(R), y_(R), z_(R)). The fourth unknown is the timecorrection s associated with the repeater, where τ_(T)=Δ+τ₁+τ_(R).

[0020] In one example, these four equations are differenced to form thefollowing three equations: $\begin{matrix}{{c\left( {m_{2} - m_{1}} \right)} = {\sqrt{\left( {x_{2} - x_{R}} \right)^{2} + \left( {y_{2} - y_{R}} \right)^{2} + \left( {z_{2} - z_{R}} \right)^{2}} - \sqrt{\left( {x_{1} - x_{R}} \right)^{2} + \left( {y_{1} - y_{R}} \right)^{2} + \left( {z_{1} - z_{R}} \right)^{2}}}} & (3) \\{{c\left( {m_{3} - m_{1}} \right)} = {\sqrt{\left( {x_{3} - x_{R}} \right)^{2} + \left( {y_{3} - y_{R}} \right)^{2} + \left( {z_{3} - z_{R}} \right)^{2}} - \sqrt{\left( {x_{1} - x_{R}} \right)^{2} + \left( {y_{1} - y_{R}} \right)^{2} + \left( {z_{1} - z_{R}} \right)^{2}}}} & (4) \\{{c\left( {m_{4} - m_{1}} \right)} = {\sqrt{\left( {x_{4} - x_{R}} \right)^{2} + \left( {y_{4} - y_{R}} \right)^{2} + \left( {z_{4} - z_{R}} \right)^{2}} - \sqrt{\left( {x_{1} - x_{R}} \right)^{2} + \left( {y_{1} - y_{R}} \right)^{2} + \left( {z_{1} - z_{R}} \right)^{2}}}} & (5)\end{matrix}$

[0021] Solving equations (3), (4) and (5) yields (x_(R), y_(R), z_(R)),the coordinates of the position of the repeater. Substituting thesecoordinates into any of the four equations represented by equation (2)yields the time correction τ_(T). The position of the repeater and thetime correction for the repeater form the calibration parameters for therepeater.

[0022] In one application, these calibration parameters are stored forsubsequent use in determining the positions of subscriber stations. Inthis application, the subscriber station receives a signal that istypically a composite of several component signals from referencesources visible to the subscriber station. The subscriber stationderives a time measurement from one of the component signals. This timemeasurement represents the time between transmission of the componentsignal by the reference source and arrival of the signal at thesubscriber station. The subscriber station also analyzes the signatureof the composite signal to determine whether the component signal wasrelayed by a repeater.

[0023] If so, a PDE in communication with the subscriber station obtainsthe calibration parameters for the repeater from a database. In oneembodiment, these calibration parameters consist of a time correctionfor, and position of, the repeater. The PDE corrects the timemeasurement using the time correction for the repeater. The correctedtime measurement then represents the amount of time required for thesignal to travel between the repeater and the subscriber station(commonly referred to as the “propagation time”). If τ_(O) refers to theoriginal time measurement, τ_(T) refers to the time correction for therepeater, and τ_(C) refers to the corrected time measurement, then thePDE derives τ_(C) by subtracting τ_(T) from τ_(O). The relationshipbetween these values can be expressed as:

τ_(C)=τ_(O)−τ_(T)   (6)

[0024] The PDE then determines the position of the subscriber stationfrom the corrected time measurement τ_(C) and the repeater position(x_(R), y_(R), z_(R)). It uses these values to determine the position ofthe subscriber station in lieu of the original time measurement τ_(O)and reference source position.

[0025] If the component signal was not relayed by a repeater, the PDEdetermines the location of the subscriber station from the uncorrectedtime measurement τ_(O) and reference source position.

[0026] Related systems are also described.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The components in the figures are not necessarily to scale,emphasis instead being placed upon illustrating the principles of thedisclosed subject matter. In the figures, like reference numeralsdesignate corresponding parts throughout the different views.

[0028]FIG. 1 is a block diagram of an embodiment of a repeater in awireless communications system relaying signals between a base stationand a subscriber station.

[0029]FIG. 2 is a block diagram illustrating the ambiguity that may beintroduced by a repeater into the process of determining the position ofa subscriber station.

[0030]FIG. 3A is a flowchart of an embodiment of a method of calibratinga repeater.

[0031]FIG. 3B illustrates an example of the format of a database recordfor storing the calibration parameters for a repeater.

[0032]FIG. 4A illustrates an example of measurement locations which maybe employed in the method of FIG. 3A.

[0033]FIG. 4B illustrates an example where the measurement locationsemployed in the method of FIG. 3A are situated along a route driven by avehicle.

[0034]FIG. 4C illustrates an example where the time measurementsemployed in the method of FIG. 3A are concurrently derived from aplurality of dispersed measurement locations.

[0035]FIG. 5 illustrates an example in which time measurements takenfrom different sets of measurement locations are used to calibratedifferent repeaters in a wireless communication system.

[0036]FIG. 6 is a timing diagram illustrating the various components ofa propagation time measurement for a signal relayed by a repeater.

[0037]FIG. 7A is a flowchart of an embodiment of a method of determiningthe position of a subscriber station in a wireless communications systememploying repeaters.

[0038]FIG. 7B is a flowchart of an implementation of the method of FIG.7A.

[0039]FIG. 8A is a block diagram of an embodiment of a system forcalibrating a repeater in a wireless communications system.

[0040]FIG. 8B is an example format of a database record which may beemployed in the system of FIG. 8A for storing one or more calibrationparameters for the repeater.

[0041]FIG. 9 is a block diagram of an implementation of a subscriberstation particularly suited for use in a wireless communications systememploying repeaters.

[0042]FIG. 10 is a diagram of an example of a position determinationsystem in which one or more calibration parameter for a repeater arestored in a database, and subsequently used for determining thepositions of subscriber stations.

[0043]FIG. 11 is a diagram illustrating calibration of a base stationaccording to an embodiment of the disclosed subject matter.

DETAILED DESCRIPTION

[0044] As used herein, the term “memory” refers to anyprocessor-readable medium, including but not limited to RAM, ROM, EPROM,PROM, EEPROM, disk, floppy disk, hard disk, CD-ROM, DVD, or the like, onwhich may be stored a series of instructions executable by a processor.

[0045] The term “processor” refers to any device capable of executing aseries of software instructions and includes, without limitation, ageneral- or special-purpose microprocessor, finite state machine,controller, computer, or digital signal processor.

[0046]FIG. 3A is a flowchart of an embodiment of a method of calibratinga repeater in a wireless communications system. In step 302, a signalrelayed by a repeater is received at a plurality of differentmeasurement locations. Step 304 follows step 302. In step 304, timemeasurements are derived from each of the measurement locations. Eachtime measurement represents the time between transmission of the signalby an originating transmitter and arrival of the signal at themeasurement location (commonly referred to as the “propagation time”).

[0047] From step 304, the method proceeds to step 306. In step 306, oneor more calibration parameters for the repeater are derived from (a) thetime measurements, and (b) the positions of the measurement locations.

[0048] This method may be further explained with reference to FIG. 4A,which illustrates an example of a wireless communications system inwhich a signal from base station 402 is transmitted over transmissionmedium 406 to repeater 404. Transmission medium 406 is any mediumcapable of transmitting the signal from transmitter 402, including byway of example and not limitation, an optical fiber, a conventionalwireline link, or a wireless link.

[0049] Repeater 404 receives the signal, amplifies it and re-transmitsit over a wireless communications link. The repeater 404 re-transmitsthe signal over a coverage area which includes measurement locations 408a, 408 b, 408 c, and 408 d. A measuring device (not shown) at each ofthe measurement locations receives the signal as transmitted by basestation 402 and relayed by repeater 404. The device then derives timemeasurements from the received signals. The time measurements arerepresentative of the time elapsed between transmission of the signal bythe base station 402 and arrival of the signal at the measurementlocation. These time measurements include the repeater self-delay anddonor base station/repeater forward link delay.

[0050] Thus, in FIG. 4A, the time measurement derived at measurementlocation 408 a is representative of the time between transmission of thesignal by base station 402, and arrival of the signal at measurementlocation 408 a. Similarly, the time measurement derived at measurementlocation 408 b is representative of the time between transmission of thesignal by base station 402, and arrival of the signal at measurementlocation 408 b. The same applies to measurement locations 408 c and 408d.

[0051] In one example, the signal relayed by a repeater is a pilotsignal that originates from a base station in a CDMA wirelesscommunications system. The pilot signal comprises a carrier signalmodulated with a repeating PN code uniquely identifying the base stationthat originated the signal. A measuring device located at a measurementlocation first tunes to the pilot channel of the CDMA system, and thenattempts to acquire the pilot signal being transmitted by the basestation.

[0052] The measuring device attempts to acquire the signal bycorrelating the received signal with the PN code for the donor basestation over a range of code phase shift hypotheses and over a range ofDoppler frequency shift hypotheses. Each correlation is performed overan integration time I, that is the product of N_(C) and M, where N_(C)is the coherent integration time, and M is the number of coherentintegrations that are non-coherently combined to form the correlationvalue.

[0053] The device then locates the peak in the correlation function thatcorresponds to a line of sight transmission by the repeater. In oneimplementation, the PN code of the donor base station is known, and themeasurement locations are located far enough from the donor base stationthat a line sight transmission from the donor base station cannot bedetected at the measuring device. The earliest “non-sidelobe” peak inthe correlation function thus represents the line of sight transmissionfrom the repeater. It will be understood by those skilled in the artthat a sidelobe is a relatively small peak occurs close to, and isrelated to, another relatively larger peak.

[0054] The device derives a time measurement from the location of theearly non-sidelobe peak along the code phase dimension of thecorrelation function. A system time reference is obtained from a GPSreceiver included in the device. The system time reference is used toadjust the time measurement so that the time measurement is in terms ofsystem time.

[0055] The positions of the measurement locations are derived by the GPSreceiver included with the device. The time measurements and positionsof the measurement locations are then provided to a positiondetermination entity (PDE). The PDE determines the calibrationparameters for the repeater 404 responsive to this information.Alternatively, the subscriber station determines its own calibrationparameters, or the collected data is stored for further processing atlater time to determine the calibration parameters.

[0056] In one implementation, the calibration parameters for therepeater 404 include a time correction for the repeater and the positionof the repeater. In this implementation, an inverse triangulationprocedure is used to derive the time correction for and position of therepeater from time measurements taken at four different measurementlocations. These four time measurements may be referred to as m_(i),1≦i≦4. The positions of the corresponding measurement locations may bereferred to as (x_(i), y_(i), z_(i)), 1≦i≦4.

[0057] Each of the time measurements represents the time betweentransmission of the signal by the originating transmitter and arrival ofthe signal at the measurement location. Included are the repeaterself-delay and donor base station/repeater forward link delay. Thus, inFIG. 6, the time measurement mi is the time between time of transmission602 and time of arrival 604, and can be expressed as:

m _(i)=Δ+τ₁+τ_(R)+τ_(2i)   (7)

[0058] where:

[0059] Δ=offset between system time and time at the originating (donor)transmitter

[0060] τ₁=the forward link delay between the originating transmitter andthe repeater

[0061] τ_(R)=the repeater self delay

[0062] τ_(2i)=the forward link delay between the repeater and receiverat measurement location i

[0063] Assuming that the position of the repeater has the unknowncoordinates (x_(R), y_(R), z_(R)), the relationship between the timemeasurement m_(i), the coordinates of the position of the repeater, andthe coordinates of the position of the measurement location i may beexpressed as follows: $\begin{matrix}{m_{i} = {\Delta \quad + \tau_{1} + \tau_{R} + {\frac{1}{c}\sqrt{\left( {x_{i} - x_{R}} \right)^{2} + \left( {y_{i} - y_{R}} \right)^{2} + \left( {z_{i} - z_{R}} \right)^{2}}}}} & (8)\end{matrix}$

[0064] where c is the speed of light.

[0065] Note that (8) represents four separate equations, one for each ofthe measurement locations. These four equations can be solved for fourunknowns. These four unknowns consist of the coordinates of the positionof the repeater (x_(R), y_(R), z_(R)), and the time correction τ_(T)associated with the repeater, where τ_(T)=Δ+τ₁+τ_(R).

[0066] In one implementation example, these four equations aredifferenced to form the following three equations: $\begin{matrix}{{c\left( {m_{2} - m_{1}} \right)} = {\sqrt{\left( {x_{2} - x_{R}} \right)^{2} + \left( {y_{2} - y_{R}} \right)^{2} + \left( {z_{2} - z_{R}} \right)^{2}} - \sqrt{\left( {x_{1} - x_{R}} \right)^{2} + \left( {y_{1} - y_{R}} \right)^{2} + \left( {z_{1} - z_{R}} \right)^{2}}}} & (9) \\{{c\left( {m_{3} - m_{1}} \right)} = {\sqrt{\left( {x_{3} - x_{R}} \right)^{2} + \left( {y_{3} - y_{R}} \right)^{2} + \left( {z_{3} - z_{R}} \right)^{2}} - \sqrt{\left( {x_{1} - x_{R}} \right)^{2} + \left( {y_{1} - y_{R}} \right)^{2} + \left( {z_{1} - z_{R}} \right)^{2}}}} & (10) \\{{c\left( {m_{4} - m_{1}} \right)} = {\sqrt{\left( {x_{4} - x_{R}} \right)^{2} + \left( {y_{4} - y_{R}} \right)^{2} + \left( {z_{4} - z_{R}} \right)^{2}} - \sqrt{\left( {x_{1} - x_{R}} \right)^{2} + \left( {y_{1} - y_{R}} \right)^{2} + \left( {z_{1} - z_{R}} \right)^{2}}}} & (11)\end{matrix}$

[0067] Solving equations (9), (10) and (11) yields (x_(R), y_(R),z_(R)), the coordinates of the location of the repeater. Substitutingthese coordinates into any of the four equations represented by equation(8) yields the time correction τ_(T).

[0068] The time correction τ_(T) for the repeater cannot be decomposedthrough the foregoing method into its constituent pieces. However, fromthe standpoint of position location determination, this does notconstitute a problem, because it is not necessary to decompose thisparameter into its constituent pieces for accurate positiondetermination.

[0069] A number of approaches are possible for collecting the timemeasurements m_(i), 1≦i≦4 and the positions (x_(i), y_(i), z_(i)), 1≦i≦4that form the inputs to the equations (8) above.

[0070] In one embodiment, illustrated in FIG. 4B, a vehicle is drivenalong a path 420. The measurement locations 408 a, 408 b, 408 c, and 408d are arbitrary locations situated along the path 420. The positions ofthese measurement locations are not known a priori.

[0071] A measuring device (not shown) accompanies the vehicle. Thedevice includes a GPS receiver. As the vehicle is driven along the path420, each of the measurement locations is successively encountered. Atime measurement is derived by the measuring device at each suchmeasurement location. In addition, a system time reference and positionof each measurement location is obtained from the GPS receiver includedin the device. The system time reference is used to adjust the timemeasurements so they are in terms of system time.

[0072] In a second embodiment, illustrated in FIG. 4C, a plurality ofsubscriber stations 422 a, 422 b, 422 c and 422 d are situated atdispersed measurement locations 408 a, 408 b, 408 c, and 408 d. A systemtime reference is derived from a GPS receiver located in the subscriberstation. In addition, the position of the measurement location isobtained from the GPS receiver. A time measurement is derived by thesubscriber station from a signal relayed by the repeater. The timemeasurement for the location is adjusted by the system time reference soit is terms of system time.

[0073] In a third embodiment, the locations of the measurement locationsare predetermined, and thus known beforehand. Fixed measuring devicesare mounted at each of the measurement locations. Each device determinesa time measurement from a signal relayed by the repeater. A system timereference is obtained and used to adjust the time measurements that areused in the calibration process.

[0074] The method of FIG. 3A may be applied to calibrate multiplerepeaters in a wireless communications system. FIG. 5 illustrates anexample of this process. Time measurements and positional informationfor each of the measurement locations 506 a(1), 506 b(1), 506 c(1), and506 d(1) are used to calibrate repeater 504 a. Similarly, timemeasurements and positional information for each of the measurementlocations 506 a(2), 506 b(2), 506 c(2), and 506 d(2) are used tocalibrate repeater 504 b.

[0075] Note that, in the particular example illustrated in FIG. 5, thereis no overlap between the measurement locations 506 a(1), 506 b(1), 506c(1), and 506 d(1) used for calibrating repeater 504 a and themeasurement locations 506 a(2), 506 b(2), 506 c(2), and 506 d(2) usedfor calibrating repeater 504 b. However, it should be appreciated thatembodiments are possible where there is complete or partial overlap inthese locations. Moreover, since the pilot signals originating from basestations 504 a, 504 b are modulated with different PN codes, a timemeasurement for both signals can be derived by the same measuring devicelocated at a single measurement location.

[0076] Referring back to FIG. 3A, in optional step 308, the one or morecalibration parameters are stored for subsequent use in determining thepositions of subscriber stations. In one implementation, the one or morecalibration parameters comprise a time correction for and position ofthe repeater. These two values are embodied as a database record that isindexed by the PN code of the donor base station. Similar records arepresent in the database for all repeaters in the network. Each of therecords is indexed with the PN code for the corresponding donor basestation. The result is an almanac for all repeaters in the network thatis updated every time a calibration procedure is performed. To ensurethat the almanac is up to date, the calibration procedure is preferablyperformed periodically or at least every time a change is made to thenetwork, such as the addition of a repeater. In one embodiment, thedatabase is accessible to a PDE that determines the positions ofsubscriber stations. In an alternative embodiment, the database isaccessible to subscriber stations that determine their own positions.

[0077]FIG. 7A illustrates an embodiment of a method for determining thelocation of a subscriber station in a wireless communications systememploying repeaters.

[0078] The method begins with step 702. Step 702 comprises deriving atime measurement from a received signal. The received signal isgenerally a composite of several component signals transmitted byreference sources visible to the receiver, but can comprise only asingle component. The time measurement is representative of the timebetween transmission of one of the component signals by thecorresponding reference source, and arrival of the signal at thereceiver.

[0079] From step 702, the method proceeds to step 704. In step 704, themethod analyzes the “signature” of the composite signal to determinewhether or not the component signal was relayed by a repeater.

[0080] In general, the “signature” of the composite signal comprises (a)the number of component signals visible to the receiver; (b)characteristics of each component, (c) the relative strength of thesesignals, and (d) the relative delay of these signals. The “signature” ofthe composite signal preferably conveys information sufficient todetermine whether a repeater relayed the component signal.

[0081] In one implementation, the component signals are pilot signals,and the “signature” of the composite signal includes: (1) the totalnumber of pilot signals visible to the receiver, (2) characteristics ofeach pilot signal, (3) their relative signal strength, and (4) theirrelative times of arrival at the receiver. The detection of other pilotsat the receiver generally identifies the donor cell as the immediatesource of the received signal. On the other hand, the lack of any otherpilots at the receiver generally identifies the repeater as theimmediate source of the component signal. A certain pattern of pilotsfrom other cells, their relative signal strength and time of arrivalscan be used to rule out or identify specific repeaters.

[0082] For example, if there is an overlap in coverage area between thedonor cell and the repeater, and the receiver is present in this area ofoverlap, the line of sight (LOS) signal received directly from the donorcell and that relayed by the repeater are marked with the same PN code.Consequently, both signals will give rise to peaks in the correlationfunction. If neither signal is subject to multi-path, the peak in thecorrelation function due to the repeater will be delayed relative tothat due to the LOS signal. The peak resulting from the repeater canthus be identified on the basis of this relative delay.

[0083] As a second example, if the LOS signal from the donor cell issubject to multi-path, it may be difficult to distinguish the peaks dueto multi-path from the peak due to the repeater. However, this ambiguitycan be resolved during the design and deployment phase by ensuring thatthe delay due to transmission through the repeater exceeds thatassociated with any multi-path produced by the RF environment. In thiscase, the repeater will leave a signature footprint in the correlationfunction in the form of a peak that is delayed beyond that due tomulti-path.

[0084] As a third example, if there is only a single peak in thecorrelation function for the PN code in question, and no other pilotsare visible to the receiver, it can be assumed that the receiver is inan area that is only accessible to signals relayed by the repeater. Thesingle peak at the PN code in question can therefore be assumed to bedue to a repeater.

[0085] As a fourth example, the calibration parameters for a particularPN code can be used to determine whether a repeater relayed a particularcomponent signal. More specifically, if the time correction τ_(T) forthe PN code greatly exceeds the corrected time measurement derived usingthis time correction, it can be assumed that a repeater relayed thecomponent signal.

[0086] Returning to FIG. 7A, from step 704, the method proceeds to step706. In step 706, the method queries whether a repeater relayed thecomponent signal. If so, step 708 is performed. In step 708, the methodcomprises obtaining one or more calibration parameters for the repeater.In one implementation, this step comprises retrieving pre-determinedcalibration parameters from a database. In one example, these values areobtained by retrieving these parameters from a database record using thePN code of the donor cell as an index to the database. In a secondimplementation, this step comprises determining these parameters “on thefly.”

[0087] From step 708, the method proceeds to step 710. Step 710comprises determining the position of the subscriber station from theone or more calibration parameters obtained in step 708.

[0088] In step 706, if the received signal was not relayed by arepeater, step 712 is performed. In step 712, the position of thesubscriber station is determined from the time measurement derived instep 702, and the position of the originating transmitter.

[0089]FIG. 7B illustrates an implementation of the method of FIG. 7B.Steps 702, 704, 706, and 712 were already explained in the context ofFIG. 7A. Only steps 720-724 are explained here.

[0090] In step 720, a time correction for and the position of therepeater are obtained. In one implementation, these parameters areretrieved from a database using the PN code of the donor cell as anindex.

[0091] From step 720, the method proceeds to step 722. Step 722comprises correcting the time measurement derived in step 714 using thetime correction for the repeater obtained in step 720. In oneimplementation, this step comprises subtracting the time correctionτ_(T) from the time measurement τ_(O) to form a corrected timemeasurement τ_(C), where τ_(C)=τ_(O)−τ_(T).

[0092] From step 722, the method proceeds to step 724. There, theposition of the subscriber station is determined from the corrected timemeasurement τ_(C) and the position of the repeater.

[0093]FIG. 8A illustrates an embodiment of a system for calibrating oneor more parameters for a repeater in a wireless communications system.The system comprises a processor 810 that is configured to determine theone or more calibration parameters for the repeater from timemeasurements and measurement location positions obtained through any ofthe previously discussed methods.

[0094] In one implementation, the processor is located within a PDE, andis configured to determine a time correction and position of therepeater by solving the four equations represented by (8) above. In thisimplementation, the processor 810 is configured to derive the parametersby executing software in the form of a series of instructions stored inmemory 812.

[0095] In one implementation, once these parameters have beendetermined, the processor 810 is configured to store them as a record indatabase 814, indexed using the PN code of the donor cell.

[0096] The record has the format shown in FIG. 8B. Field 816 is the PNcode of the donor cell. Field 818 is the time correction for therepeater. Field 820 is the position of the repeater.

[0097]FIG. 9 is a block diagram of a subscriber station that isparticularly suited for use in a wireless communications systememploying repeaters.

[0098] Processor 902 is configured to execute software instructions, andmemory 904 is configured to hold the software instructions and data thatare accessible by the processor 902.

[0099] Persistent storage 906 is configured to hold provisioninginformation useful for acquiring wireless communications services, andcan be implemented as a combination of devices such as a non-volatileEEPROM combined with a SIM card.

[0100] Keypad 908 and display 910 are both typically provided as part ofa user interface. Similarly, microphone 912 and speaker 914 are bothtypically provided to support use of the device for receiving andtransmitting voice.

[0101] Radio transceiver (Tx/Rx) 916 is provided for receiving andtransmitting information over a wireless communications link. Modem 918is provided for modulating baseband information, such as voice or data,onto an RF carrier, and demodulating a modulated RF carrier to obtainbaseband information. Antenna 922 is provided for transmitting amodulated RF carrier over a wireless communications link and receiving amodulated RF carrier over a wireless communications link.

[0102] Correlator 920 is provided for deriving correlation functionsfrom a received signal comprising a composite of pilot signalstransmitted by various reference sources visible to the subscriberstation. For a given PN code, the correlator 920 derives a correlationfunction by correlating the received signal with the PN code over arange of code phase shift hypotheses and a range of Doppler frequencyshift hypotheses. It then locates a predetermined number of the peaks ofthe correlation function.

[0103] Processor 902 is configured to analyze this information todetermine the earliest non-sidelobe peak of the correlation function. Ifsuch a peak is detected, the processor 902 is also configured to derivea time measurement from the location of this peak in the code phasedimension. If a system time reference is available, the processor 902adjusts the time reference using the system time reference so that thetime measurement is in terms of system time.

[0104] Processor 902 is also configured to analyze the signature of thereceived signal to determine if the peak relates to a pilot signalreceived directly from the reference source or if it relates to a signalthat was relayed by a repeater. This process was previously described inrelation to FIG. 7A.

[0105]FIG. 10 illustrates an example of a system for determining theposition of a subscriber station in a wireless communication systememploying repeaters. Subscriber station 1002 receives signalstransmitted by a plurality of reference sources 1004 a, 1004 b, 1004 c,and 1004 d, visible to the receiver in the subscriber station. Asillustrated, the reference sources may be BTSs, GPS satellites, orcombinations of BTSs and GPS satellites.

[0106] Each of the reference sources transmits a signal that ismodulated with an identification code that uniquely identifies thereference source. In one implementation, the identification codes are PNcodes that may differ in length or periodicity according to thereference source involved. For IS-95 compliant CDMA systems, the PN codeis a sequence of 32,768 chips that is repeated every 26.67 msec. Incurrent GPS systems, the PN code is a sequence of 1,023 chips. Thesequence is repeated every one millisecond.

[0107] The signals transmitted by reference sources 1004 a, 1004 b, and1004 c, are all received directly by the subscriber station 1002. Thus,all are line of sight signals. However, the signal transmitted by basestation 1004 d is relayed by repeater 1006, and this is not a line ofsight signal from the standpoint of the donor cell 1004 d.

[0108] A database 1010 is accessible to position determination entity(PDE) 1008. The database 1010 contains entries for each of the repeatersthat are present in the wireless communications system. Each of theentries comprises a record that contains the position of and timecorrection for the corresponding repeater. This information is derivedusing any of the methods for calibrating a repeater that have beenpreviously discussed. Each entry is indexed using the PN code of thedonor cell.

[0109] The subscriber station 1002 is equipped with a correlator that,in conjunction with related software executable by a processor withinthe subscriber station, is configured to derive a time measurement foreach of the pilot signals. If a system time reference is available, thesubscriber station 1002 uses this information to adjust the timemeasurements so they are in terms of system time. Alternatively, thistask is performed by the PDE 1008.

[0110] The subscriber station 1002 then communicates the timemeasurements to PDE 1008. Upon receipt of this information, PDE 1008checks the signature of the composite signal to determine if any of thepilot signals were relayed by a repeater. The process of analyzing thesignature of a composite signal to determine the immediate origin of acomposite signal was previously discussed in relation to the method ofFIG. 7A.

[0111] If a time measurement derived from a signal relayed by a repeateris present, then the PDE 1008 uses the PN code for the donor cell toretrieve the calibration parameters for the repeater from the database1010. In particular, the PDE 1010 retrieves a record containing the timecorrection for, and position of, the repeater. It then uses the timecorrection for the repeater to correct the time measurement. It alsosubstitutes the position of the repeater for that of the donor cell. Itperforms these adjustments for each of the time measurements determinedto involve signals relayed by a repeater.

[0112] It then determines the position of the subscriber station 1002using the corrected time measurements and updated positions. Again, inone implementation, known triangulation procedures are used to derivethe position of subscriber station 1002. Once determined, the positionof the subscriber station 1002 may be communicated by the PDE 1008 tothe subscriber station 1002 or some other network entity.

[0113] Alternatively, the subscriber station 1002 determines its ownposition using the database of calibration parameters that is accessibleby the subscriber station 1002.

[0114] While various embodiments of the disclosed subject matter havebeen described, it will be apparent to those of ordinary skill in theart that many more embodiments and implementations are possible.

[0115] In particular, embodiments are possible in which network entitiesother than repeaters may be calibrated through application of themethods described. For example, FIG. 11 illustrates the method of FIG.3A applied to the process of calibrating a base station. Referring toFIG. 6, the time correction for a base station will comprise thecomponent referred to as Δ, the offset between system time and time asmaintained at the base station, but does not include the componentsreferred to as τ₁ or τ_(R). Other than this difference, the approach forcalibrating the base station is identical to that described previouslyin relation to a repeater.

[0116] One or more signals are transmitted from BTS 1102 over a wirelesscommunications link. The transmission occurs over a coverage area thatincludes measurement locations 1104 a, 1104 bb, 1104 cc, and 1104 d. Areceiver (not shown) at each of the measurement locations receives asignal as transmitted by BTS 1102.

[0117] A time measurement is derived from the signal received at each ofthe measurement locations. In one implementation, the time measurementis representative of the travel time between BTS 1102 and the receiver(commonly referred to as the “propagation time”). Thus, in the exampleillustrated in FIG. 11, the time measurement derived at measurementlocation 1104 a is representative of the propagation time from BTS 1102to measurement location 1104 a. Similarly, the time measurement derivedat measurement location 1104 b is representative of the propagation timefrom BTS 1102 to measurement location 1104 b. The same applies tomeasurement locations 1104 c and 1104 d.

[0118] One or more calibration parameters for BTS 1102 are thendetermined responsive to the time measurements and the locations of themeasurement locations. In one implementation, the calibration parametersfor BTS 1102 comprise a time correction for and position of BTS 1102.These parameters are determined from the time measurements and positionsof the measurement locations using the equations (8) referred topreviously.

[0119] Embodiments are also possible in which the calibration parametersare used for determining the positions of subscriber stations in anyposition determination system, including without limitation terrestrialsystems, network-based or subscriber station based terrestrial systems,GPS satellite systems, or hybrids thereof. Moreover, any method ofposition determination may be employed, including without limitationAOA, TOA, cell ID, with or without TA or RTD enhancements, E-OTD, OTDOA,or A-GPS, with or without IPDL, TA-IPDL, or OTDOA-PE modifications.

[0120] Accordingly, the invention is not to be restricted except by theappended claims.

What is claimed is:
 1. A method of calibrating a repeater in a wirelesscommunications system comprising: receiving a signal, transmitted by anoriginating transmitter and relayed by a repeater, at a plurality ofdifferent measurement locations; deriving time measurements from each ofthe received signals, each time measurement representative of the timebetween transmission of the signal by the originating transmitter andarrival of the signal at the measurement location; and deriving one ormore calibration parameters for the repeater from (a) the timemeasurements, and (b) the positions of the measurement locations.
 2. Themethod of claim 1 wherein the one or more calibration parameterscomprise a time correction for the repeater, and the position of therepeater.
 3. The method of claim 1 further comprising storing the one ormore calibration parameters.
 4. The method of claim 1 wherein theplurality of different measurement locations comprise four or moredifferent measurement locations.
 5. The method of claim 1 wherein thetime measurements are sequentially derived from signals sequentiallyreceived at successive ones of the measurement locations.
 6. The methodof claim 1 wherein the time measurements are concurrently derived fromsignals concurrently received at the measurement locations.
 7. Themethod of claim 1 wherein the measurement locations are fixed andpredetermined.
 8. The method of claim 1 the positions of the measurementlocations are sequentially derived using a GPS receiver sequentiallyplaced at successive ones of the measurement locations.
 9. The method ofclaim 2 wherein the time measurements are obtained at four differentmeasurement locations and may be referred to as m_(i), 1≦m≦4, each ofthe measurement locations has a position with coordinates (x_(i), y_(i),z_(i)), and the time correction τ_(T) for the repeater and thecoordinates (x_(R), y_(R), z_(R)) of the position of the repeater arederived by solving for four equations that may be represented as:${m_{i} = {\tau_{T} + {\frac{1}{c}\sqrt{\left( {x_{i} - x_{R}} \right)^{2} + \left( {y_{i} - y_{R}} \right)^{2} + \left( {z_{i} - z_{R}} \right)^{2}}}}},{1 \leq i \leq 4}$

where c is the speed of light.
 10. The method of claim 3 wherein the oneor more calibration parameters are stored in a database.
 11. The methodof claim 10 wherein the database includes one or more calibrationparameters for each of a plurality of repeaters.
 12. The method of claim5 wherein the time measurements are sequentially obtained by drivingaround an area with a measuring device, and using the measuring deviceto sequentially obtain the measurements at successive ones of themeasurement locations.
 13. The method of claim 12 wherein the measuringdevice includes a GPS receiver, and the positions of the measurementlocations are sequentially obtained by using the GPS receiver in themeasuring device to sequentially obtain the positions of successive onesof the measurement locations.
 14. The method of claim 6 wherein the timemeasurements are concurrently obtained from a plurality of dispersedmeasuring devices.
 15. The method of claim 14 wherein the measuringdevice includes a GPS receiver, and the positions of the measurementlocations are concurrently obtained from the GPS receivers included inthe plurality of dispersed measuring devices.
 16. The method of claim 1wherein the signals received at the measurement locations all originatefrom the same transmitter.
 17. A method of determining the position of asubscriber station in a wireless communications system employingrepeaters comprising: receiving a composite signal at the subscriberstation, the composite signal having a signature and comprising one ormore component signals; determining from the signature of the compositesignal whether a component signal was relayed by a repeater; obtainingone or more calibration parameters for the repeater if the componentsignal was relayed by the repeater; and determining the position of thesubscriber station based at least in part on the one or more calibrationparameters for the repeater.
 18. The method of claim 17 wherein the oneor more calibration parameters comprise a time correction for therepeater, and the position of the repeater.
 19. The method of claim 17wherein the one or more calibration parameters are retrieved from adatabase.
 20. The method of claim 19 wherein the database includes oneor more calibration parameters for a plurality of repeaters.
 21. Themethod of claim 17 wherein the component signal originates with areference source.
 22. The method of claim 21 wherein the referencesource is a base station in a wireless communications system.
 23. Themethod of claim 21 wherein the reference source is a GPS satellite. 24.The method of claim 21 wherein the one or more calibration parametersare retrieved using a PN code for the reference source.
 25. The methodof claim 24 wherein the one or more calibration parameters comprise atime correction τ_(T) for the repeater and position of the repeater. 26.The method of claim 21 wherein a time measurement τ_(O) is derived fromthe component signal, the time measurement representing the time betweentransmission of the signal by the reference source, and receipt of thesignal by the subscriber station, and this time measurement is correctedusing the time correction τ_(T) for the repeater to form a correctedtime measurement τ_(C), where τ_(C)=τ_(O)−τ_(T).
 27. The method of claim26 wherein the position of the subscriber station is determined from thecorrected time measurement τ_(C) and position of the repeater in lieu ofthe time measurement τ_(O) and position of the reference source.
 28. Themethod of claim 17 wherein the signature of the composite signalincludes the number of component signals visible to the subscriberstation.
 29. The method of claim 28 wherein the signals are all pilotsignals.
 30. The method of claim 17 wherein the signature of thecomposite signal includes the relative strength of the component signalsvisible to the subscriber station.
 31. The method of claim 17 whereinthe signature of the composite signal includes the relative time ofarrivals of the composite signals visible to the subscriber station. 32.A system for determining the position of a subscriber station in awireless communications system employing repeaters comprising: areceiver for receiving a composite signal at the subscriber station, thecomposite signal have a signature and comprising one or more componentsignals; a processor for determining from the signature of the compositesignal whether a component signal was relayed by a repeater; and anentity for obtaining one or more calibration parameters for the repeaterif the component signal was relayed by the repeater, and determining theposition of the subscriber station based at least in part on the one ormore calibration parameters for the repeater.
 33. The system of claim 32wherein the one or more calibration parameters for the repeater comprisea time correction for the repeater, and the position of the repeater.34. The system of claim 32 wherein the entity is configured to retrievethe one or more calibration parameters from a memory.
 35. The system ofclaim 34 wherein the entity is configured to retrieve the one or morecalibration parameters from a database.
 36. The system of claim 35wherein the one or more calibration parameters for the repeater are inthe form of a record in a database that is accessible by the processorusing the PN code for the originating transmitter of the signal as anindex.
 37. The system of claim 33 wherein the entity is configured tocorrect a time measurement derived from the signal using the timecorrection for the repeater, and determine the position of thesubscriber station at least in part from the corrected time measurementfor and the position of the repeater.
 38. The system of claim 33 whereinthe entity is a position determination entity.
 39. The system of claim33 wherein the entity is a subscriber station.
 40. A memory tangiblyembodying a sequence of software instructions configured to determine(a) a time correction τ_(T) for an entity, and (b) the coordinates(x_(R), y_(R), z_(R)) of the entity from (1) time measurements obtainedat four different measurement locations that may be referred to asm_(i), 1≦i≦4, and (2) the coordinates (x_(i), y_(i), z_(i)), 1≦i≦4, ofthe measurement locations, by solving for four equations that may berepresented as:${m_{i} = {\tau_{T} + {\frac{1}{c}\sqrt{\left( {x_{i} - x_{R}} \right)^{2} + \left( {y_{i} - y_{R}} \right)^{2} + \left( {z_{i} - z_{R}} \right)^{2}}}}},{1 \leq i \leq 4}$

where c is the speed of light.
 41. The memory of claim 40 wherein theentity is a base station, and the time measurements are derived fromsignals transmitted by the base station and received at the fourdifferent measurement locations.
 42. The memory of claim 40 wherein theentity is a repeater, and the time measurements are derived from signalstransmitted by an originating transmitter, and relayed by the repeater,prior to receipt thereof at the measurement locations.
 43. A systemcomprising a processor and the memory of claim 41, wherein the processoris configured to access and execute the software instructions tangiblyembodied by the memory.
 44. A method of calibrating a repeater in awireless communications system comprising: driving a vehicle along aroute with a measuring device located on or within the vehicle;successively encountering a plurality of measurement locations havingalong the route; using the device to receive a signal, transmitted by anoriginating transmitter and relayed by a repeater, at each of themeasurement locations; using the device to derive a time measurementfrom the received signal at each of the measurement locations; using aGPS receiver included with the device to determine a system timereference; using the system time reference to adjust thetimemeasurements to produce adjusted time measurements, an adjusted timemeasurement for a measurement location representing the time, in termsof system time, between transmission of the signal by an originatingtransmitter and arrival of the signal at the measurement location; usingthe GPS receiver to determine the position of each of the measurementlocations; and deriving a time correction for and position of therepeater from (a) the adjusted time measurements, and (b) the positionsof the measurement locations.
 45. A subscriber station comprising: areceiver for receiving a signal having a signature; a correlator forderiving a correlation function from the signal, and identifying one ormore peaks in the correlation function; a processor configured to derivea time measurement from a peak in the correlation function, analyze thesignature of the signal to determine if the peak relates to a signalthat was relayed by a repeater, and provide an indication whether thepeak relates to a signal that was relayed by a repeater.
 46. A method ofcalibrating an entity in a wireless communications system comprising: astep for receiving a signal, transmitted by an originating transmitterand relayed by a repeater, at each of a plurality of differentmeasurement locations; a step for deriving time measurements from eachof the received signals, each time measurement representative of thetime between transmission of the signal by the originating transmitterand arrival of the signal at a measurement location; and a step forderiving one or more calibration parameters for the entity from (a) thetime measurements, and (b) the positions of the measurement locations.47. The method of claim 46 wherein the entity is a repeater.
 48. Themethod of claim 46 wherein the entity is a base station.
 49. A systemfor determining the position of a subscriber station in a wirelesscommunications system employing repeaters comprising: means forreceiving a composite signal at the subscriber station, the signal havea signature and comprising one or more component signals; means fordetermining from the signature of the composite signal whether acomponent signal was relayed by a repeater; and means for obtaining oneor more calibration parameters for the repeater if the component signalwas relayed by the repeater, and determining the position of thesubscriber station based at least in part on the one or more calibrationparameters for the repeater.